This invention relates generally to heating devices for water heaters; and more particularly to a heat pump used to heat the water in a water heater. Heat pumps have been used before to heat water for water heating installations. Examples of these uses are illustrated in the following United States Patents:
______________________________________ Class/ U.S. Pat. No. Issued Inventor Subclass ______________________________________ 2,575,325 11/1951 Ambrose et. al. 62/238 E 2,668,420 2/1954 Hammell 62/238 E 3,922,876 12/1975 Wetherington, Jr., et. al. 62/238 EX 4,073,285 2/1978 Wendel 62/238.6 4,136,731 1/1979 DeBoer 165/12 4,141,222 2/1979 Ritchie 62/238 E 4,142,379 3/1979 Kuklinski 62/238.6 4,330,379 5/1982 Robinson, Jr. 62/181 ______________________________________
Typically early prior art heat pumps for water heaters employed thermally operated flow control valves to restrict the rate of water flow through the heat pump to assure that the outlet water reached a sufficiently high temperature so that the heated water could be returned to the top of a water storage tank and be available for immediate use. Usually, water was drawn from the bottom of the tank through the dip tube on the cold water inlet at the top of the tank which extends down to the lower end of the tank. The heated water was returned to the top of the tank after being heated. A water circulating pump associated with the heat pump was required to provide a flow rate of approximately 2 GPM per 12,000 BTUH to maintain sufficient heat transfer to extract the heat from the condenser without exceeding the condensing temperature limit of the compressor as the water approached its final tank temperature.
The prior art flow control valves were very similar to the thermostat in an automobile radiator system. A bleed hole allowed a small amount of water to flow through the heat pump condenser heat exchanger when the water from the water tank was cold. This allowed the flow control valve to sense the temperature of the water leaving the heat pump. Typically, the valve would begin to open as the water approached about 115.degree. F. and was fully open at about 125.degree. F. During an initial tank heat up or after a batch of hot water was withdrawn, the flow control valve would modulate the water flow rate to maintain an outlet temperature of approximately 120.degree. F. until the entire tank began to heat up. As the water entering the heat pump from the bottom of the tank began to warm, the output of the heat pump would raise the temperature of the water higher than 120.degree. F. causing the flow control valve to open further to increase the flow rate until the maximum flow rate was reached. The system continued to operate until the tank was heated to its set point as controlled by the tank thermostat, usually about 140.degree. F.
The advantage of this system was that it heated the water tank from the top down making some hot water instantaneously available before a tank was completely heated to an acceptable temperature. Unfortunately, this type flow control valve experienced serious reliability problems from corrosion, scaling, and plugging. Other types of flow control valves were also found to either be too expensive and/or unreliable to be practical. Because of the problems, this concept of using variable flow control was about abandoned in the mid-1980's.